/Engine/Plugins/Runtime/MeshModelingToolset/Source/ModelingComponents/Public/Selection/MeshTopologySelectionMechanic.h
Bydefault, the shift key will cause new clicks to add to the selection. However, this can be changed by supplying a different function to check here.
void SetShouldAddToSelectionFunc ( [TFunction](API\Runtime\Core\GenericPlatform\TFunction)< bool(void)> Func )